1. Field of the Invention
The present invention relates to an acceleration sensor and, more particularly, to an acceleration sensor suitable for detecting a large change in the speed of a vehicle caused by a collision and the like.
2. Description of the Related Art
An acceleration sensor of this kind is described in U.S. Pat. No. 4,827,091. This known sensor comprises a cylinder made of a conductive material, a magnetized inertial member slidably mounted in the cylinder so as to be movable in the longitudinal direction of the cylinder, a conductive member fixed at least on an end surface of the inertial member facing one longitudinal end of the cylinder, a pair of electrodes fixed at one longitudinal end of the cylinder, which are caused to conduct via the conductive member when the conductive member of the magnetized inertial member makes contact with the electrodes, and an attracting member fixed relative to the cylinder near the other longitudinal end of the cylinder and made of such magnetic material that magnetically attracts the inertial member.
In this acceleration sensor, the magnetized inertial member and the attracting member attract each other and when no or almost no acceleration is applied to the sensor, the inertial member is at rest at the other end in the cylinder.
If a relatively large acceleration acts on this acceleration sensor, the magnetized inertial member moves against the attracting force of the attracting member. During the movement of the inertial member, an electrical current is induced in this cylinder, producing a magnetic force which biases the inertial member in the direction opposite to the direction of movement of the inertial member. Therefore, the magnetized inertial member is braked, so that the speed of the movement is reduced.
When the acceleration is less than a predetermined magnitude, or threshold value, the magnetized inertial member comes to a stop before it reaches the front end of the cylinder. Then, the inertial member is pulled back by the attracting force of the attracting member.
When the acceleration is greater than the predetermined magnitude, or the threshold value, e.g., the vehicle carrying this acceleration sensor collides with an object, the inertial member reaches at one end of the cylinder. At this time, the conductive layer on the front end surface of the inertial member makes contact with both electrodes to electrically connect them with each other. If a voltage has been previously applied between the electrodes, an electrical current flows when a short circuit occurs between them. This electrical current permits detection of collision of the vehicle.
FIG. 2 is a perspective view illustrating these electrodes 40 and 42, FIG. 3 is a cross-section view taken along III--III line in FIG. 2 and FIG. 4 is an enlarged view of the main part of FIG. 3. These electrodes 40 and 42 are formed as parts of conductive pieces 46 and 48, respectively, which are stamped from sheet metal. The conductive pieces 46 and 48 have terminals 50 and 52, respectively, with which lead wires (not shown) are connected. An electrical resistor 54 is bridged between the conductive pieces 46 and 48 and lead electrodes 54a and 54b of the electrical resistor 54 are soldered to the conductive pieces 46 and 48, respectively.
The conductive pieces 46 and 48 which are connected together through the electrical resistor 54 are insert-molded out of a synthetic resin together with the resistor 54. The resistor 54 and main portions of the conductive pieces 46 and 48 are buried in a holding member called a contact holder.
In the prior art acceleration sensor, however, the front end surface of the magnetized inertial member tends to hit the edge portions of the electrodes 40 and 42 when it hits the electrodes 40 and 42. It is because there is a small gap between the outer periphery of the magnetized inertial member and the inner periphery of the cylinder, and the front end surface of the magnetized inertial member slants slightly from a plane perpendicular to the axial line of the cylinder. The gap is about 0.3 mm when the inner diameter of the cylinder is 7 mm and the length of the magnetized inertial member is 12 mm. Due to that, the front end surface of the magnetized inertial member can slant from the plane perpendicular to the axial line of the cylinder by the gap of 0.3 mm. And due to that slant, the front end surface of the magnetized inertial member hits the edge portions 41 of the electrodes 40 and 42.
When the front end surface of the magnetized inertial member hits the edge portions of the electrodes 40 and 42, a contact pressure of the magnetized inertial member and the electrodes 40 and 42 becomes unstable, causing an unstable contact resistance of the magnetized inertial member and the electrodes 40 and 42.
As shown in FIG. 4 (a partially enlarged view of the edge portion 41 in FIG. 3), the electrodes 40 and 42 are stamped from a clad in which a surface layer 3 made of a gold-silver alloy (e.g. silver content is about 8 wt. %) having a thickness of 1 to 2 micrometer is clad to the surface of a base material 1 composed of a copper alloy (e.g. a copper-beryllium alloy) having a thickness of about 80 micrometer through the intermediary of a substrate layer 2 made of silver-palladium alloy (e.g. palladium content is about 60 wt. %) having a thickness of 2 to 5 micrometer. The reason why such gold alloy is clad is to enhance the corrosion resistance of the electrode surface. However, the substrate layer 2 and surface layer 3 tend to become deficient at the edge of the electrodes 40 and 42 when stamped as shown in FIG. 4. Accordingly, if the magnetized inertial member hits the edge portion where such gold alloy surface layer 3 is deficient, the contact resistance of the magnetized inertial member and the electrodes 40 and 42 becomes unstable.